Remove test for `PREVENT_AUTH_USER_WRITES` since code breaks in Django 2 (#23536)

...and it turns out we don't need the switch anymore, anyway.

When we upgraded to Django 1.11, this flag was added in order to
allow for a database migration that would render the user table
unwriteable for up to half an hour:
https://github.com/edx/edx-platform/pull/17561

This involved swapping out the signal handler for logins via
`user_logged_in.disconnect(django_update_last_login)`, but with
Django 2.0, that disconnect is silently failing (returning
false). Likely the disconnect is now happening too soon.
(See edx-platform/common/djangoapps/student/apps.py line 21 in 61e1eda2.)

The result is that by the time the waffle switch is consulted, the
normal handler has already run, and the user's last login date has
already been updated.

For now we're just removing the test, and have filed ARCHBOM-1084 for
followup (deleting the switch and related code).

openedx/core/djangoapps/user_authn/views/tests/test_login.py

@@ -179,19 +179,6 @@ class LoginTest(SiteMixin, CacheIsolationTestCase):
         self._assert_response(response, success=True)
         self._assert_audit_log(mock_audit_log, 'info', [u'Login success', unicode_email])
 
-    def test_last_login_updated(self):
-        old_last_login = self.user.last_login
-        self.test_login_success()
-        self.user.refresh_from_db()
-        assert self.user.last_login > old_last_login
-
-    def test_login_success_prevent_auth_user_writes(self):
-        with waffle().override(PREVENT_AUTH_USER_WRITES, True):
-            old_last_login = self.user.last_login
-            self.test_login_success()
-            self.user.refresh_from_db()
-            assert old_last_login == self.user.last_login
-
     def test_login_fail_no_user_exists(self):
         nonexistent_email = u'not_a_user@edx.org'
         response, mock_audit_log = self._login_response(